1. Field of the Invention
The present invention relates to an optical scanning device, and particularly relates to an excellent optical scanning device which is employed in a digital copier, a laser beam printer or the like and which carries out simultaneous writing with a plurality of laser beams.
2. Description of the Related Art
In FIG. 2 of Japanese Patent Application Laid-Open (JP-A) No. 2001-215423, an optical scanning device which scans with a multi-beam irradiated from a multi-spot laser diode is disclosed. Further, in FIG. 3 of the same publication, a multi-spot laser diode is disclosed.
The multi-spot laser diode is a vertical cavity surface emitting laser (VCSEL) in which a plurality of light emitting spots are arranged in two dimensions. In the multi-spot laser diode, a total of thirty-six light emitting spots are disposed two-dimensionally with predetermined spacings, six in a main scanning direction by six in a sub-scanning direction.
FIG. 20 of the present application is a block diagram showing an example of a driving circuit of a multi-spot laser diode. Each laser diode in a array is independently driven with video signals by independent video interface circuits A and B and LD drivers A and B.
The optical scanning device described in the aforementioned JP-A No. 2001-215423 is provided with a photodiode. This photodiode receives a portion of a light beam emitted from the laser array and transmits light amount signals to an unillustrated light amount control circuit, and thus light amounts of the beam are controlled so as to attain a target value.
A laser beam irradiated from the laser array is scanned on a surface (that is, a scanning surface) of a photoreceptor, which is provided to be rotatable about a central axis thereof, by a multi-beam scanning optical system. The multi-beam scanning optical system is formed by a collimator lens, an aperture which limits the diameter of a beam from the collimator lens, a cylinder lens having optical power only in the sub-scanning direction, a mirror, a polygon mirror, an f-θ lens, a first cylinder mirror and a second cylinder mirror. Herein, scanning of the beam in accordance with rotation of the polygon mirror is referred to as main scanning, and scanning due to movement of the scanning surface in a direction intersecting the main scanning direction is referred to as sub-scanning.
In this structure, an unillustrated mirror and an unillustrated photosensor are provided at a position corresponding to an end portion of the photoreceptor, for detecting a position of the scanning beam in the main scanning direction. A timing for output of image signals to the laser array is set by a beam position detection signal outputted from the photosensor.
FIG. 21 is a diagram showing an exposure profile which is formed in the sub-scanning direction by the laser beams on the scanning surface in a case of scanning writing using this optical scanning device. Here, a number of laser beams n=36, and a case of progressive scanning is shown. That is, neighboring beams are scanned along respectively neighboring scanning lines of a raster image.
The optical scanning device carries out simultaneous scanning of 1st to 36th scanning lines using a 1st laser beam to a 36th laser beam at a time of a 1st cycle of scanning of the beams (represented by a scan number (j=1) in the drawing). Then the optical scanning device carries out simultaneous scanning of 37th to 72nd scanning lines with the laser beams for scan number (j=2). Thereafter, scanning of blocks of 36 lines in order of scan numbers (j=3), (j=4), . . . is similarly recursively carried out in the same manner.
FIG. 22 is a diagram describing a conventional technique for writing with interlaced scanning. This technique is described in, for example, Japanese Patent No. 3,237,452. Herein, a natural number i (≧2), which is a spacing r between two neighboring beams divided by a scanning line spacing p, is defined as an “interlaced scanning period”. Note that i=1 for progressive scanning.
For example, if the scanning period i=(r/p)=3, that is, if the spacing r between two beams is specified to be 3p (p being the scanning line spacing), then for the scan number (j=1), a 1st scanning line is formed by a 1st laser beam and a 4th scanning line is formed by a 2nd laser beam. Then, when the scan number (j=2), a 3rd scanning line is formed by the 1st laser beam and a 6th scanning line is formed by the 2nd laser beam. In the same way, for scan number (j=3) and onward, 5th, 7th, 9th, . . . scanning lines are formed by the first laser beam and 8th, 10th, 12th, . . . scanning lines are synchronously formed by the second laser beam.
At this time, if a number of beams n=2, the beam spacing r=62.5 μm and the scanning line spacing p=20.83 μm, then the following equation holds.
                              1          /                      (                          n              ·              p                        )                          =                ⁢                  1          /                      (                          2              ×              0.02083                        )                                                  =                ⁢                  24.0          ≧                      4            ⁢                                                  ⁢            line            ⁢                                                  ⁢            pairs            ⁢                          /                        ⁢            mm                              
It has been written that, because irregularities of light amounts that are caused by multi-beam scanning are not discernible by the human eye, excellent images can be obtained.
In the optical scanning device represented in FIG. 21, a beam diameter d (of 1/e2 intensity) is around 65 μm, and is large in comparison to the scan spacing p (approx. 10.6 μm). Thus, overlapping of neighboring beams is large when progressive scanning is carried out. Therefore, an exposure profile resulting from scan number (j=1) is as shown by (A) in FIG. 21, and an exposure profile resulting from (j=2) is as shown by (B). Here, respective vicinities in a vicinity from a 5th scanning line to a 30th scanning line are exposed by a plurality of beams acting together. A time difference between exposure timings of cooperating beams is up to two μs.
In contrast, an exposure portion between the 36th scanning line and the 37th scanning line is exposed by scan number (j=1), and is then exposed again by the first laser beam for scan number (j=2) after an interval of around 300 μs. Thereafter, scanning for scan numbers (j=3, 4, . . . ) recurs in the same manner, and the exposure profiles (C), (D), . . . are successively formed.
Now, an electric charge amount generated by a photoreceptor is proportional to an amount of absorbed light, that is, to a product of exposure intensity and exposure duration, and this is generally referred to as a reciprocity law. However, in a case of exposure with extremely short periods, amounts of variation in electric potential are small compared with a case of exposure over relatively long periods, even if the total light amounts are equivalent between the two cases. Thus, reciprocity failures occur.
FIGS. 23A and 23B are diagrams for explaining reciprocity law failures. FIG. 23A is a diagram showing states of electrons and a positive hole when light is incident at a photoreceptor. FIG. 23B is a diagram showing an electrical charge generation amount in relation to irradiation time difference. In a case in which a time difference between irradiations at the photoreceptor is short, charge density of a charge generation layer is reduced and, as shown in FIG. 23B, a recombination rate of electrons and holes is reduced. Thus, reciprocity law failures occur.
As a result of such reciprocity law failures, optical density between the 36th scanning line and the 37th scanning line as shown in FIG. 21 is more dense than at other regions, and a density fluctuation appears with a period of the number of beams n.
For example, in the case of an optical scanning device which is structured with the number of beams being 36 and the scanning line spacing p=21.16 μm, banding with a period of 0.762 mm is generated. Converted to a spatial frequency, this is 1.31 (cycle/mm). Hereafter, this banding is referred to as “banding due to reciprocity law failures”.
FIG. 24 is a diagram showing a characteristic of contrast sensitivity at the human eye against spatial frequency (herebelow referred to as the visual transfer function (VTF) of the human eye). According to the VTF of the human eye, the human eye can easily distinguish density fluctuations when a spatial frequency of an image is around 1 cycle/mm, but it is more difficult to distinguish images with a high frequency, of 3 cycle/mm or above.
However, in the case of the aforementioned optical scanning device with the number of beams being 36 and the scanning line spacing p=21.16 μm, banding of 1.31 cycle/mm due to reciprocity law failures occurs. As can be seen from FIG. 24, this banding can easily be distinguished by human visual characteristics, and image quality is adverse affected.